The performance of gas turbine engines, whether measured in terms of efficiency or specific output, is improved by increasing the turbine gas temperature. In modern engines, the high pressure (HP) working gas temperatures are hotter than the melting point of the material of the blades and vanes, necessitating internal air cooling of these airfoil components. During its passage through the engine, the mean temperature of the gas stream decreases as power is extracted. Nonetheless, in some engines, the intermediate pressure (IP) and low pressure (LP) turbines are also internally cooled.
FIG. 1 shows an isometric view of a typical HP stage of a cooled turbine. Cooling air flows are indicated by arrows.
HP turbine nozzle guide vanes 1 (NGVs) consume the greatest amount of cooling air on high temperature engines. HP blades 2 typically use about half of the NGV flow. The IP and LP stages downstream of the HP turbine use progressively less cooling air.
The HP turbine airfoils are cooled by using high pressure air from the compressor that has by-passed the combustor and is therefore relatively cool compared to the working gas temperature. Typical cooling air temperatures are between 800 and 1000 K, while working gas temperatures can be in excess of 2100 K. Cooling air is carried along internal conduits within the airfoils and exits through a large number of cooling holes formed in the surfaces of the airfoils.
The cooling air bled from the compressor that is used to cool the hot turbine components is not used fully to extract work from the turbine. Therefore, as extracting coolant flow has an adverse effect on the engine operating efficiency, it is important to minimise the use of cooling air and maximise its effectiveness.
HP turbine NGVs are cooled using a combination of internal convection and external film cooling to protect the metal from overheating and oxidation attack. The film cooling air is bled onto the gas washed surfaces of the aerofoil through numerous rows of relatively small diameter cooling holes. These small holes are susceptible to blockage from the inside, by air-borne dirt particles or from compressor rub-seal material. In addition the holes can become blocked from the outside, by dirt particles and combustion products, that attach themselves to the gas washed surfaces of the NGV, particularly the leading edge and pressure surfaces.
Engines on aircraft that operate out of or over sandy areas such as desert are particularly susceptible to sand ingestion and cooling hole blockage.
Partially blocked film cooling holes substantially reduce the quantity of cooling air passed by the cooling system, which causes the NGV to overheat, loss of the NGV thermal barrier coating (TBC), and ultimately premature low cycle fatigue cracking and oxidation of the gas washed surfaces of the NGV. Eventually the NGVs will burn away, particularly at the leading and trailing edges, causing the turbine performance to deteriorate and early component failure.
By fan shaping the cooling holes at their downstream ends the holes become less susceptible to external blockage. However, the holes still remain prone to internal blockage. For example, the film cooling holes in the NGVs are normally produced by a laser drilling process in which the metal of the NGV is vaporised to create the holes. When a drilled hole breaks through to the internal cavity of an NGV, a rough ring of solidified metal remains, and this rough edge tends to catch the dirt particles and a build-up occurs restricting the entrance to the hole.
Air-born dirt and contaminants tend to be in greatest concentration towards the outer walls of the flow chamber that contains the combustor flame tube. This is due to the centrifugal swirl that exists at the exit of the HP compressor. Thus the cooling air that is supplied to the NGV cooling scheme from the radially outer combustor bypass annulus generally contains a higher concentration of dirt, dust and contaminants compared with the radially inner combustor bypass annulus. It follows that an option for cooling scheme locations that may be particularly susceptible to dirt blockage is to have their cooling air sourced from the inner annulus. However, sourcing large quantities of coolant from the in-board feed results in higher cooling air flow velocities, which reduces the static pressure of the cooling air and necessitates higher feed pressure delivery from the compressor in order to meet the backflow pressure margin to prevent hot gas ingestion. Further, increasing the pressure drop across the combustor has an adverse affect on turbine efficiency and specific fuel consumption.
Another option is to increase the diameter of the film cooling holes. However, this increases the quantity of cooling air required by the holes and therefore reduces the turbine efficiency of the component.
Although NGVs can be particularly susceptible to cooling hole blockage, other aerofoils (rotor blades or static vanes) with similar cooling holes in the turbine section of a gas turbine engine can also experience cooling hole blockage.